Special Physics Photo of the Week

 Physics Photo of the Week

April 28, 2023 - A Volcano's Mouth

A recent trip to Hawai'i Volcano's National Park on the Island of Hawaii gave me and Vicki a rare opportunity to look into the most active volcano on Earth - the crater of Kilauea Volcano.  (Pronounced "kilou-way-ah").

It's amazing that a national park lets visitors venture close to a volcano - a mountain that semi-periodically erupts red-hot molten lava of temperatures hovering 1000 deg Celsius.  Volcanoes have wreaked havoc and death on a number of major cities throughout the world with their highly destructive powers.

The photo at right shows a close-up of the inner crater of the Kilauea crater.  This active caldera is called the Halema'uma'u crater (pronounced "hall-lay-ma-oo-ma-oo").  This last erupted earlier this year - January-February of 2023.  See photos the eruption including visitors watching from about the vantage point of the photo at right ( https://www.nps.gov/havo/learn/nature/2023-summit-eruption.htm )

The photo at left shows the two largest volcanoes on Hawai'i: Mauna Loa on the left and Mauna Koa on the right.  Notice the rounded dome shapes that resemble shields.   I recorded this photo from the  commercial airline window as we were leaving Hawai'i.  These do not resemble the conical strato volcanoes.

Why is Kilauea volcano relatively safe to visit as a national park - even though it is one of the most active volcanoes in the world?  The answer to this paradox is that the volcanoes on the island of Hawai'i ("The Big Island") are "shield" volcanoes, not "strato" volcanoes that are scattered around the world including the cascade mountains in western North America.  Strato volcanoes are formed at the junction of major plate boundaries on the Earth where much geophysical activity takes place.  The lavas emitted by strato volcanoes are much more viscous (resistant to flowing) than the more basaltic shield volcanoes.  Strato volcanoes have been formed by continental plate collisions where the Earth's crust of a continent or ocean is subducted - or buried - below the neighboring continental plate.  The subducted plate was forced to move and become buried deep in the Earth's crust and reach the hot mantle where the material melts. The continents are composed of mostly silicates (granites, quartz, etc).  The oceanic crust of the Earth is mostly more dense iron-bearing minerals called basalts.  When basalts melt at high temperatures, they are much less viscous than the melted silicates.  Thus when the basalts melt and come to the Earth's surface they flow further and spread out more than the the more viscous melted silicates.  The oceanic hot-spot volcanoes on Hawai'i erupt melted basalt which spreads out large distances.  The volcano is a broad dome that resembles a warrior's shield.  In contrast, the strato volcanoes of the cascades are formed from mostly melted silicates that are more viscous, require more pressure to erupt, and give a highly conical shape to the mountaiin.  As a result eruptions of the strato volcanoes are much more violent and destructive from the increased pressure that is required to form an eruption.

An analogy may be seen in different cooking appliances: a "Crock-pot" that has a glass lid that is held in place by gravity, and a pressure cooker that has a tight seal and well-fitted clamped-on lid to build up a high pressure and temperature.  When a stew is cooked in a crock-pot the slight pressure from the steam forces the lid to slowly rise and give an occasional "burp" that easily lets off the slightly pressured steam.  A pressure cooker has a regulator that maintains a high pressure - thus creating a higher pressure to raise the temperature of the contents.  If the pressure regulator became stuck or a safety valve does not release - the explosion produced is very dangerous and could be lethal.  Thus the crock-pot resembles a shield volcano, while a pressure-cooker resembles a strato volcano with its high pressure potential destructive power.

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  Physics Photo of the Week is published occasionally during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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